Interference between electronic products and components mainly comes from themselves or outside. Their operations in certain degree are affected by interference, and thus it is important as to whether they themselves generate the electronic noises or have a shield to avoid the interference of foreign electronic noises. Noise interferences mainly can be classified into electrostatic discharge (ESD), electromagnetic interference (EMI), and radio frequency interference (RFI). These interferences are highly likely to cause a short circuit or a damage in an electronic component. As the density of the circuits in the electronic component increases, the interference problems become more severe. As a result, the electrostatic or electromagnetic interference shielding is required for more and more electronic products and components.
The uses of conductive polymeric composite materials mainly include four areas according to their conductivity for different resistance requirements, which are:
First is an anti-electrostatic material, second is an ESD shielding material, third is EMI/RFI shielding material, and fourth is a conductive connection material. In general, a polymeric material is a good insulator with a surface resistance of greater than 1012 Ω/cm2, and is very easy to have electrical charges agglomerated on the surface thereof. A direct modification to the polymer such as blending the polymer with polyaniline, or an addition of a conductive filler (e.g. conductive carbon black or metal powder) can increase the conductivity of the polymer to reduce the charge agglomeration or enhance transferring of electrical charges. The anti-electrostatic material has a lower rate of charge transfer, which can avoid the charge agglomeration and the occurrence of spark. As the rate of charge transfer is faster, it becomes a conductive polymeric composite material. Usually, a polymeric material is blended with a conductive filler or an antistatic agent to achieve antistatic or EMI shielding effects.
The conductance of an ordinary material, according to its surface resistance, can be classified into the following: high conductivity, static electricity dissipation, and antistatic. (1) A conductive material for EMI/RFI shielding needs to have a volume resistance of less than 1 Ω·cm, as well as be applicable in electrostatic protection at a high voltage. (2) A material with a surface resistance of 105-109 Ω/cm2 is defined as an static electricity dissipation material and is often used in the protection of short circuit for an electronic facility liable for static electricity agglomeration or electric arc formation. Its static electricity dissipation rate is slightly lower than that of a conductive material, and such a material is often used as a packaging material or an operation tool. (3) A material with a surface resistance of 109-1012 Ω/cm2 is an antistatic material, which provides an ESD protection or antistatic property at a low voltage, and is often used as a packaging material.
A human body and materials such as plastics, synthetic fibers, and glass fibers, are easy to generate high voltage static electricity, for example, a plastic material, during its use, production, and transportation processes, is easy to generate static electricity. The formation of static electricity can be prohibited by the addition of a conductive carbon black or an antistatic amine additive. An amine or other antistatic agent also plays other roles, such as a lubricant, and a mold release agent, etc.
There are two types of antistatic mechanisms: one is lubricating effect, i.e. reducing the frictional force; the other is the conduction of static charges or absorption of moisture, allowing the charges to disappear in the atmosphere, thereby achieving the antistatic effect. Generally speaking, the most popular additives of antistatic material are amines, amine salts, and polyvinyl alcohol. The main antistatic mechanism of these additives is absorbing moisture in the atmosphere, thereby achieving the antistatic effect.
Several defects of an antistatic agent must be taken into consideration, for examples, carbon black tends to release from the matrix material and form contamination due to wearing off, and an amine antistatic agent usually does not function well in a dry environment. Therefore, currently researches are being carried out on developing permanent type antistatic agents and temperature independent antistatic agents, which will not be significantly affected by moisture and temperature.